1. Field of the Invention
The present invention relates generally to apparatus for distributing alternating current electrical energy to a load wherein the electrical energy originates in a direct current energy source. More particularly, the invention pertains to an electrical distribution system in which a switching element in an inverter is controlled to provide an alternating current waveform having a low harmonic content and wherein the same switching element may also be controlled to provide regulation of the alternating current load voltage.
2. Description of the Prior Art.
It is widely recognized that apparatus for converting electrical energy from direct current sources to low frequency alternating current and voltage for distribution to a substantial load should have several desirable characteristics. In order to minimize unnecessary heating in certain types of load elements by higher harmonic currents, it is desirable that the A. C. voltage have a waveform which is a good approximation to a pure sine wave. In order to accomodate transients and large reactive currents in the load, it is desirable that the power source have a low impedance. It is also desirable to regulate the load voltage to remain at a relatively constant amplitude. Where semiconductor elements are used for the conversion from direct to alternating current and for the regulation of the voltage, it is desirable to operate these elements at the lowest possible frequency.
In one well-know class of distribution systems, an inverter produces a square wave output which is then gated through a bidirectional switch, the switch being operated to vary the width of the bipolar pulses of the square wave to provide regulation. Considerable filtering is required in this type of system to produce a sinusoidal output. Filter components are, in general, cumbersome and expensive at power frequencies. The presence of filters also increases the source impedance.
In another well-known type of distribution system which comprises a closed-loop system tracking the waveform of a sinusoidal reference signal, a power switch is clocked to operate at a relatively high frequency in a bang-bang fashion to produce a train of pulse-width modulated bipolar power pulses. This approach reduces the filtering requirements, along with the source impedance, since the lower frequency harmonics are reduced but it requires the use of fast-response semiconductor elements.
In yet another well-known type of distribution system, a plurality of pulse gating signals are generated to control the operation of a plurality of power switches. The outputs of the plurality of power switches are added. The gating signal and corresponding power switch outputs, when added, approximate a sine wave. Regulation is achieved by means of a controller which adds pulses, deletes pulses, or shifts their time of occurence, as necessary. While this type of system can eliminate the requirement for filtering and tends to have a low source impedance, it can cause discrete discontinuities in the waveform. In addition, the controller tends to be complex.
Lastly, distribution system is which a plurality of quasi-square waves are added to produce an approximation to a sine wave are known. It has been the practice to regulate the output of such systems by adjusting the direct current input to the inverters. Such systems have a high dynamic source impedance and, therefore, a poor dynamic response.